1. Field of the Invention
The present invention generally relates to ultra-high pressure discharge lamps and, more particularly, to novel methods of lighting such ultra-high pressure discharge lamps, ultra-high pressure discharge lamps to which the methods of the invention are applicable, and ballasts for use in such ultra-high pressure discharge lamps.
2. Description of the Related Art
In these years, a liquid crystal projector market has grown rapidly, and a market for consumer-oriented data projectors and rear projection television sets adapted for digital broadcasting is expected to grow explosively in near future. In meeting the demand of such a growing market, the life of a light source and countermeasures against flicker are critical factors.
Heretofore, brightness has been the only one standard for estimating the performance of such a light source and, hence, there has been a keen competition for a higher brightness. Ultra-high pressure discharge lamps developed as a result of such a keen competition generally exhibit a high luminance attained by shortening the arc length to the limit and increasing the pressure of mercury in the discharge bulb to 150 atm or higher so as to raise the arc temperature.
Though the luminance of such a conventional ultra-high pressure discharge lamp has been improved in the manner described above, tradeoffs have occurred such that an electrode surface adjacent the location at which an arc is generated is consumed too rapidly and that flicker due to such rapid consumption of the electrodes is likely to occur. At present, these tradeoffs cannot be controlled. Conventional ultra-high pressure discharge lamps cause steady flicker to occur after 400 hours"" use (refer to Table 1), and the illuminance of a screen illuminated by a projector incorporating such a conventional ultra-high pressure discharge lamp lowers by about 50% after 1,000 hours"" use of the lamp. Thus, conventional ultra-high pressure discharge lamps also have a problem in keeping the screen illuminance.
Though there has been developed an ultra-high pressure discharge lamp assuring a certain illuminance kept for 1,000 hours or longer, an electrode surface adjacent the arc generating location in such a lamp becomes roughened and, hence, the discharge initiating point frequently moves about, which is observed as flicker on the screen. It is, therefore, needless to say that light sources to be incorporated in consumer-oriented products have to be flickerless while keeping a satisfactory screen illuminance for a longer time.
An invention related to an ultra-high pressure discharge lamp is disclosed in Japanese Patent Gazette No. 2,829,339. The Gazette mentions use of a typical halogen cycle for preventing blackening due to evaporation of tungsten and describes that if the amount of a halogen used is large, the electrodes are consumed heavily and, hence, the life of the lamp becomes as short as several hundreds of hours. The invention of this Gazette consists in reducing the amount of the halogen to the limit in order to prevent such a rapid consumption of electrodes due to the halogen. However, such an extremely reduced amount of the halogen is critical and hence may fail to ensure a desired halogen cycle. As a result, the discharge bulb is sometimes blackened due to evaporation of tungsten.
The Patent Gazette teaches the fact that the life of 5,000 hours or longer was attained by reducing the amount of the halogen to the limit. Even an ultra-high pressure discharge lamp that escaped being blackened at its discharge bulb, however, was observed to have its electrodes consumed or roughened at their surfaces adjacent the arc generating location. In an extreme case plural projections were formed on the electrodes. Due to such roughness at the surfaces of the electrodes, the discharge initiating point frequently moved about on the electrode surface roughened and such a movement was reflected as flicker on a screen. Thus, the light source according to the Patent Gazette was not a practically acceptable one as a light source having a service life rated at 5,000 hours which assures a service life of 5,000 hours presently demanded.
As described above, the Patent Gazette mentions the amount of a halogen as the cause of the occurrence of electrode consumption. Specifically, the Patent Gazette concludes that encapsulation of a halogen in an amount of 1xc3x9710xe2x88x924 xcexcmol/mm3 or more is the only factor dominating the electrode consumption. In the experiment conducted by the inventors of the present invention, however, it was observed that an ultra-high pressure discharge lamp encapsulating a halogen in an amount less than 1xc3x9710xe2x88x924 xcexcmol/mm3 was blackened due to an incomplete halogen cycle while at the same time an electrode surface adjacent the arc generating location was deformed (roughened or formed with projections) after lapse of a few hundreds of hours from the starting of the experiment, that the discharge initiating point began moving about after lapse of 500 hours, and that flicker appeared on a screen after lapse of 1,000 hours. From this experiment it is deduced that other factors than the amount of a halogen which dominate the consumption of electrodes and the roughening of an electrode surface exist.
Japanese Patent Gazette No. 2,980,882 discloses an invention that intends to prevent blackening or blurring of a discharge bulb and consumption of electrodes by encapsulating an increased amount of a halogen as large as 2xc3x9710xe2x88x924 to 7xc3x9710xe2x88x923 xcexcmol/mm3 in order to absorb and reduce short wavelength ultraviolet ray. According to this Gazette, the mechanism of the occurrences of such inconveniences is that short wavelength ultraviolet ray generated during lighting of the lamp cuts off the silicon (Si)-oxygen (O) bond of quartz glass forming the discharge bulb of the lamp thereby to cause SiO to evaporate and such evaporated SiO in turn causes blurring of the discharge bulb and consumption of the electrodes. Stated otherwise, the Patent Gazette concludes that short wavelength ultraviolet ray is the factor dominating the blackening or blurring of a discharge bulb and consumption of electrodes.
The inventors of the present invention actually constructed an ultra-high pressure discharge lamp according to the scope claimed by the latter Patent Gazette and verified the effect of this lamp. As a result, it was found that consumption of the electrodes proceeded unusually rapidly and this fact was far worse than stated in the Gazette. More specifically, it was observed in the ultra-high pressure discharge lamp in which a large quantity of a halogen was encapsulated in order to inhibit the generation of short wavelength ultraviolet ray that the electrodes were consumed or roughened at their surfaces adjacent the arc generating location after lapse of 100 hours from the starting of the experiment, while at the same time the discharge initiating point moved around frequently, and that flicker appeared on a screen after lapse of 400 hours from the starting of the experiment. It is concluded from these results that the life of the ultra-high pressure discharge lamp according to the Patent Gazette in question cannot reach even 2,000 hours, which is the shortest life presently required of projector light sources and that it is absolutely impossible for the lamp to have a life of 10,000 hours, which is required of applications in television. This experiment also proved that the life of an ultra-high pressure discharge lamp could not be prolonged by absorbing and reducing short wavelength ultraviolet ray with an increased amount of an encapsulated halogen.
In view of the foregoing, the inventors of the present invention conducted experiments on the assumption that factors dominating the consumption or roughening of electrodes and formation of projections on the electrodes in an ultra-high pressure discharge lamp operated with alternating or direct current include a thermal factor, an oxygen factor and a silicon factor. As a result, it was discovered that the primary factor is oxygen remaining in the discharge bulb and the secondary factor is the thermal factor.
In the experiment elucidating the thermal factor, it was checked whether or not an improvement was made by increasing the thermal capacity of each electrode (refer to Table 1). According to the results of the experiment, an electrode having a higher thermal capacity was less consumed and the arc length was inhibited to lengthen. This can be confirmed from data of variations in arc length relative to a lighting time parameter. It was, however, found that no improvement was made as to roughening of an electrode surface adjacent the arc generating location. Thus, it is impossible to make the discharge initiating point stationary at one point. Incidentally, roughening of an electrode surface was observed by transmission of X-rays.
The experiment elucidating the oxygen factor and the silicon factor began with analysis of the state of electrodes in a finished lamp by means of EDX (Energy Dispersive X-ray spectroscopy). The EDX analysis proved that silicon is not a dominant factor. Since oxygen in a great quantity was detected on the surfaces of tungsten electrodes and the presence of tungsten oxides such as WO and WO2 and mercury oxide was observed, oxygen is considered to be a dominant factor. According to the investigation made as to when oxygen was brought into the lamp and left therein, it was found that oxygen was brought into the lamp in the second sealing process, which is the final sealing process.
In conclusion, however, it is impossible to prevent production of SiO, which is responsible for the production of oxygen in the second sealing process, so long as the lamp envelope is made of quartz and, hence, it is impossible to provide a flickerless and long-life light source exhibiting a high luminance.
The present invention intends to solve this very difficult problem by utilizing the results of the experiments conducted to elucidate the factors dominating the life of an ultra-high pressure discharge lamp and the occurrence of roughening at surfaces of the electrodes of the lamp. Accordingly, it is an object of the present invention to provide a method of lighting an ultra-high pressure discharge lamp, which enables the lamp to have a longer life and exhibit a higher luminance. Another object of the present invention is to provide an ultra-high pressure discharge lamp to which the method of the invention is applicable. Still another object of the present invention is to provide a ballast for use in such a lamp, and a lighting system incorporating the lamp and the ballast.
According to the present invention, there is provided a method of lighting an ultra-high pressure discharge lamp having a discharge bulb made of quartz glass in which a pair of electrodes are disposed facing each other, the method comprising:
lighting the lamp with alternating current; and
providing a time period of at least one second for which the alternating current assumes a lighting frequency of 5 Hz or lower during a period for which the lamp lights, wherein
the electrodes each have a current withstanding capability against a current 1.5 times or more as high as a current value at which the lamp lights stably.
According to the present invention, there is also provided a method of lighting an ultra-high pressure discharge lamp having a discharge bulb made of quartz glass in which a pair of electrodes are disposed facing each other, the method comprising:
lighting the lamp with direct current; and
performing a control such that a time period of at least one second for which the direct current is applied in a direction opposite to a direction of the current applied in usual lighting is intermittently provided during a period for which the lamp lights, wherein
a negative electrode of the pair of electrodes has a current withstanding capability against a current 1.2 times or more as high as a current value at which the lamp lights stably.
From the results of experiments (to be described later) conducted by the inventors of the present invention, it seems that roughening of an electrode surface occurs due to an intricate correlation between a synergistic effect of the thermal factor and the oxygen factor and the mass of each electrode used. In the case where the mass of the electrode is small relative to the lighting current value, impingement of thermal electrons upon the electrode causes the temperature at an electrode surface adjacent the arc generating location to rise unusually and the highest temperature portion of the electrode surface is deformed or consumed by fusion with growing projections formed around the deformed portion. This is the thermal factor.
The oxygen factor is given rise to as follows. Oxygen in the discharge bulb resulting from decomposition of SiO produced in the final (second) sealing process reacts with tungsten forming the electrodes to produce tungsten oxide on surfaces of the electrodes. Further, tungsten oxide thus produced reacts with a halogen to produce a halogenated tungsten oxide, which in turn is decomposed by arc, resulting in tungsten partially deposited and accumulated on an end portion of each electrode. The tungsten thus deposited and accumulated on the electrodes gradually forms plural projections. Since thermal electrons have a disposition to be emitted from the tip of each projection, the discharge initiating point frequently changes due to the roughened electrode surfaces or the presence of plural projections, thus resulting in flicker. Stated otherwise, the occurrence of flicker is more dependent upon the surface state of the negative electrode from which thermal electrons are emitted than that of the positive electrode upon which thermal electrons impinge.
Since the lamp envelope is made of quartz as described above, oxygen is inevitably produced and, hence, roughening of an electrode surface due to the oxygen factor cannot but be considered to be inevitable. Thus, the present invention has overcome the problem by combining use of electrodes having an increased thermal capacity (i.e., an increased mass) and an improvement in applying electric current to the lamp. Specifically, in the case of lighting with alternating current, the lighting frequency of the current is established so that a time period of at least one second for which the alternating current assumes a lighting frequency of 5 Hz or lower is provided during a period for which the lamp lights. By so doing, thermal electrons are allowed to flow from one electrode to the other continuously for more than ⅕ second, which is longer than the alternating period of the current for usual lighting, whereby the surface of the thermal electron receiving electrode is heated and slightly fused. It should be noted that since the usual lighting frequency for an ultra-high pressure discharge lamp is 50 Hz or higher, the alternating period of the current in usual lighting is {fraction (1/50)} second or shorter. If the surface of the thermal electron receiving electrode is roughened or formed with projections, the roughened surface or projections are fused and thinly extended over the electrode due to surface tension, whereby the roughening or projections are eliminated and, hence, the original spherical electrode surface is restored. Stated otherwise, if the lamp lights at a lighting frequency of 5 Hz or higher, the direction of the current applied changes before the roughening or projections on the electrode surface have been eliminated. As a result, the electrode surface becomes more roughened or the projections grow larger with time.
However, if the mass of each electrode is too small, the temperature of the electrode becomes too high due to continuous application of the current and, hence, the electrode is consumed. In view of this, the present invention has a feature that the electrodes each have a current withstanding capability against a current 1.5 times or more as high as a current value at which the lamp lights stably, whereby the electrodes can be prevented from being consumed due to continuous application of the current and allowed to restore their surfaces. Thus, the ultra-high pressure discharge lamp for lighting with alternating current can be made flickerless and to have a longer life.
In the case of lighting with direct current, a control is performed such that a time period of at least one second for which the direct current is applied in a direction opposite to a direction of the current applied in usual lighting is intermittently provided during a period for which the lamp lights. By doing so, thermal electrons are temporarily emitted from the positive electrode to the negative electrode to restore the negative electrode by heating. In the case of lighting with direct current, the negative electrode has a smaller mass than the positive electrode. However, the negative electrode needs to have a current withstanding capability against a current 1.2 times or more as high as a current value at which the lamp lights stably in order to withstand continuous application of the current for the predetermined time period described above. In this way, the ultra-high pressure discharge lamp for lighting with direct current also can be made flickerless and to have a longer life.
As described above, an electrode surface become roughened or formed with projections by the thermal factor and the oxygen factor. Since the positive electrode is heated due to constant impingement of thermal electrons thereupon in usual lighting, such roughening or projections on the positive electrode caused by the two factors can be eliminated by heat resulting from continuous application of the current and, hence, the surface of the positive electrode is restored.
The negative electrode, however, continuously emits thermal electrons and, hence, surface restoration, which occurs on the positive electrode, does not occur on the negative electrode. Accordingly, the negative electrode is gradually roughened or formed with growing projections. To avoid the occurrence of this inconvenience, the aforementioned feature that a control is performed such that a time period of at least one second for which the direct current is applied in a direction opposite to a direction of the current applied in usual lighting is intermittently provided during a period for which the lamp lights, allows thermal electrons to impinge upon the negative electrode for a predetermined time period thereby causing the surface of the negative electrode to be restored. At least one second is required as this electrode surface restoring time period. If it is shorter than one second, insufficient restoration will result.
In the case of lighting with alternating current, the term xe2x80x9ccurrent withstanding capabilityxe2x80x9d means a capability such that each electrode is not deformed at an end portion thereof during a time period of 10 seconds for which the electrode is applied with a current 1.5 times as high as a current value at which the lamp lights stably. In the case of lighting with direct current, on the other hand, the term xe2x80x9ccurrent withstanding capabilityxe2x80x9d means a capability such that each electrode is not deformed at an end portion thereof during a time period of 10 seconds for which the electrode is applied with a current 1.2 times as high as a current value at which the lamp lights stably.
According to the present invention, there is also provided a method of lighting an ultra-high pressure discharge lamp having a discharge bulb made of quartz glass in which a pair of electrodes are disposed facing each other, the method comprising:
lighting the lamp with alternating current; and
providing a time period of at least one second for which the alternating current assumes a value is equal to or higher than a rated current value during a period for which the lamp lights steadily, wherein
the electrodes have a current withstanding capability against a current 1.5 times or more as high as a current value at which the lamp lights stably.
According to the present invention, there is also provided a method of lighting an ultra-high pressure discharge lamp having a discharge bulb made of quartz glass in which a pair of electrodes are disposed facing each other, the method comprising:
lighting the lamp with direct current; and
performing a control such that a time period of at least one second for which the direct current is applied in a direction opposite to a direction of the current applied in usual lighting is intermittently provided during a period for which the lamp lights, while a time period of at least one second for which the alternating current assumes a value equal to or higher than a rated current value when the lamp lights steadily, wherein
a negative electrode of the pair of electrodes has a current withstanding capability against a current 1.2 times or more as high as a current value at which the lamp lights stably.
In these methods, an excessive current, which is equal to or higher than a rated current, is applied for at least one second during a period for which the lamp lights steadily, whereby an electrode surface can be rapidly restored.
According to the present invention, there is also provided an ultra-high pressure discharge lamp configured to be lighted with alternating current, comprising a discharge bulb made of quartz glass and imposed with a bulb load of 0.7 w/cm2 or more, and a pair of electrodes disposed facing each other in the discharge bulb, the discharge bulb encapsulating therein at least one halogen selected from the group consisting of I, Br, and Cl in an amount ranging between 1xc3x9710xe2x88x924 xcexcmol/mm3 and 1xc3x9710xe2x88x922 xcexcmol/mm3, mercury in an amount of 0.15 mg/mm3 or more, and a rare gas as an initiator gas, wherein
the electrodes each have a current withstanding capability against a current 1.5 times or more as high as a current value at which the lamp lights stably.
According to the present invention, there is also provided a ballast for use in an ultra-high pressure discharge lamp configured to be lighted with alternating current as recited above, the ballast being capable of providing a time period of at least one second for which the alternating current assumes a lighting frequency of 5 Hz or lower during a period for which the lamp lights.
In an embodiment of the ballast stated above, the time period of at least one second for which the alternating current assumes a lighting frequency of 5 Hz or lower is provided during a time period of three minutes from a time point at which lighting of the lamp is initiated to a time point at which lighting of the lamp becomes completely stabilized.
In another embodiment of the ballast, the time period of at least one second for which the alternating current assumes a lighting frequency of 5 Hz or lower is provided at a time point at which a lamp voltage lowers to a predetermined value during a period for which the lamp lights stably.
Yet another embodiment of the ballast is capable of providing a time period of at least one second for which the alternating current assumes a value equal to or higher than a rated current value during a period for which the lamp lights.
According to the present invention, there is also provided an ultra-high pressure discharge lamp configured to be lighted with direct current, comprising a discharge bulb made of quartz glass and imposed with a bulb load of 0.7 w/cm2 or more, and a pair of electrodes disposed facing each other in the discharge bulb, the discharge bulb encapsulating therein at least one halogen selected from the group consisting of I, Br, and Cl in an amount ranging between 1xc3x9710xe2x88x924 xcexcmol/mm3 and 1xc3x9710xe2x88x922 xcexcmol/mm3, mercury in an amount of 0.15 mg/mm3 or more, and a rare gas as an initiator gas, wherein
a negative electrode of the electrodes has a current withstanding capability against a current 1.2 times or more as high as a current value at which the lamp lights stably.
According to the present invention, there is also provided a ballast for use in an ultra-high pressure discharge lamp configured to be lighted with direct current as recited above, the ballast being capable of performing a control such that a time period of at least one second for which the direct current is applied in a direction opposite to a direction of the current applied in usual lighting is intermittently provided during a period for which the lamp lights.
In an embodiment of the ballast stated above, the time period of at least one second for which the current is applied in a direction opposite to a direction of the current applied in usual lighting is provided during a time period of three minutes from a time point at which lighting of the lamp is initiated.
In another embodiment of the ballast, the time period of at least one second for which the current is applied in a direction opposite to a direction of the current applied in usual lighting is provided at a time point at which a lamp voltage lowers to a predetermined value during a period for which the lamp lights stably.
Yet another embodiment of the ballast is capable of providing a time period of at least one second for which the direct current assumes a value equal to or higher than a rated current value during a period for which the lamp lights.
According to the present invention, there is also provided a lighting system comprising an ultra-high pressure discharge lamp configured to be lighted with alternating current as recited above, and a ballast for use in such a lamp.
According to the present invention, there is also provided a lighting system comprising an ultra-high pressure discharge lamp configured to be lighted with direct current as recited above, and a ballast for use in such a lamp.
These and other objects, features and attendant advantages of the present invention will be fully appreciated from the following detailed description taken in conjunction with the attached drawings.